The invention relates to a method of making a seamless hot-finished steel pipe.
Following the invention by the brothers Mannesmann to produce a thick-walled hollow tubular ingot from a heated billet, many different proposals have been suggested to stretch this hollow tubular ingot in a same hot-working step at same temperature. Keywords include the continuous rolling process, the rotary-forged process, the piercing mill process, and the Pilger step-by-step rolling process (Stahlrohr-Handbuch [Steel Pipe Handbook], 10. ed; Vulkan-Verlag Essen, 1986, III. Manufacturing Processes).
All mentioned processes have their benefits for different size ranges and materials, whereby combinations are possible as well. The continuous rolling process and the piercing mill process are applicable for the size range of 5″ to 18″, the Pilger-mill process is applicable for the size range of up to 26″. When a thicker wall in the range of >30 mm is involved, the continuous rolling process and the piercing mill process are less suitable while the Pilger-mill process, although not encountering any problems with the wall thickness, exhibits a production cycle that is slower. A drawback common to all mentioned processes is the more or less long modification times during a change in size.
The three stages piercing- stretching- reduction-rolling are a characteristic for the production of seamless pipes from a heated billet (H. Biller, Das Walzen nahtloser Rohre—Probleme der Verfahrensauswahl [Rolling of Seamless Tubes—Problems of Process Selection], Stahl und Eisen 106 (1986), No. 9, pages 431-437).
For some time, attempts have been made to save a step in order to lower production and assembly costs. These attempts have shown little success to date.
DE 1 906 961 A1 discloses a method of making seamless tubes from hollow bodies produced by continuous casting. In this known process, the cast strand is divided and the respective section is initially stretched with the assistance of an internal tool and rolling by hot forging. Thereafter, the pre-stretched section is rolled to a tube (shell) by a continuous rolling train, and a finished pipe is made thereform through subsequent stretch-reduction. This proposed process should be applied for mass production of pipes of small diameter from hollow bodies made through continuous casting. The proposal is intended to overcome the problem of excessive strain of the skew rolls during initial stretching.